A VCSEL is a semiconductor laser that emits light in a vertical direction with respect to a substrate. In general VCSEL has an active region with a large gain, a low threshold current, high optical power, reliability, and adequately controlled polarization. Since the VCSEL does not require a cleavage process, it allows to be integrated into two-dimensional arrays for on-wafer testing. It is suitably used in various consumer applications such as the light source of an image forming apparatus, the light source of an optical pickup device, the optical communication data transmitter of optical interconnections and optical modules, etc.
The active region of the VCSEL arranged between two semiconductor multilayer reflector (DBR: Distributed Bragg Reflector, for example) mirrors is the region in which electrons and holes combine to generate light. The active region includes a quantum-well structure provides the photonic device with a lower threshold current, a high efficiency and a greater flexibility in choice of emission wavelength.
A quantum-well structure is composed of at least one (n) quantum-well layer interleaved with a corresponding number (n+1) of barrier layers. Each of the quantum well layers has a thickness in the range from about one nanometer to about ten nanometers. The barrier layers are typically thicker than the quantum well layers. The semiconductor materials of the layers of the quantum-well structure depend on the desired emission wavelength of the photonic device. The semiconductor material of the barrier layers differs from that of the quantum-well layer, and has larger bandgap energy and a lower refractive index than that of the quantum well layer.
A quantum-well structure composed of gallium arsenide (GaAs) quantum well layers and aluminum gallium arsenide (AlGaAs) barrier layers has been proposed for the active region of a conventional VCSEL to generate light with a wavelength of 850 nm. FIG. 1a is an energy-band diagram of an exemplary active region 10 incorporating such a quantum-well structure. Band energy is plotted as ordinate and distance from the substrate is plotted as abscissa. As shown, the active region 10 includes a first cladding layer 121, a first barrier layer 141 made of AlGaAs, a quantum-well layer 16 made of GaAs, a second barrier layer 142 made of AlGaAs, and the second cladding layer 121. The energy-band diagram of FIG. 1a shows the energies of the conduction band 101 and valence band 102 of the semiconductor material of each of the layers just described.
The active region 10 composed of the barrier layers 141 and 142 of AlGaAs and the quantum-well layer 16 of GaAs has a Type I heterostructure. In this heterostructure, the energy of the valence band of GaAs of the quantum-well layer 16 is greater than the energy of the valence band of the AlGaAs of the barrier layers 141 and 142, but the energy of the conduction band of GaAs of the quantum-well layer 16 is less than the energy of the valence band of the AlGaAs of the barrier layers 141 and 142.
The line-up of the band energies in a quantum-well structure having a Type I heterostructure confines electrons 156 to the conduction band 101 of the quantum-well structure 16 and confines holes 158 to the valence band 102 of the quantum-well structure 16. As a result, the electron-hole recombination process takes place between carriers confined in the same layer.
However, the conduction band and valence band discontinuities (ΔEc and ΔEv) between GaAs quantum well and AlGaAs barrier layers are small due to their fixed bandgap values, thus carrier leakage will happen in the quantum well layers 16 from the quantum well layer across the barrier layers as shown in FIG. 1b which brings a reduced performance, such as the optical confinement in the region layer is low, the high temperature performance is reduced, and the reliability characteristics of the VCSELs causes a degraded life time due to internal self heating effect. Besides above, in the moisture/humidity ambient the element Al in the barrier layer can easily be oxidized and can form defects or dislocations causing further reduction of the life time of VCSELs.
U.S. Pat. No. 8,837,547 B2, US Publication No. 2014/0198817 A1, and US Publication No. 2012/0236891 disclose 850 nm wavelength VCSEL respectively, but all of them has the drawbacks mentioned above more or less.
Thus, it is desired to provide an improved VCSEL structure with increased optical confinement and increased speed, good high temperature and reliability characteristics and long life time to overcome the above-mentioned drawbacks.